additional transmute and transmute_unchecked harnesses

[AlexLB99]

Toward solving #19

This pr adds some additional harnesses for transmute_unchecked and transmute.

With this, we suspect that the main harnesses for transmute/transmute_unchecked are there (besides adding more of the same kinds of harnesses, like the 2-way harnesses for more types). We go into more detail about what needs to be done for part 1 of challenge 1 (verifying the transmute intrinsics directly), as well as what has been done and what can't be done here: https://docs.google.com/document/d/1zFGANNMx8mZ8fucKrN--ELwKASUPeP20THH6M_fQ7jg/edit?usp=sharing

Just one note: transmute has far fewer harnesses than transmute_unchecked here -- this is because it is not currently possible to write a wrapper for transmute, and it is thus not possible to write a function contract. A lot of the harnesses for transmute_unchecked test the function contract's validity clause, rather than the function itself, explaining this disparity.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 and MIT licenses.

[AlexLB99]

Would be to happy hear your thoughts on this (@celinval, @feliperodri, or anyone else interested in this challenge). In particular, we would like to hear what you think about the completeness of part 1 of challenge 1 (i.e., is there anything major that Kani can do that isn't currently being verified here?). For our own thoughts on this, we refer to the above linked completeness document. Thank you!